Where Will the New Horizons Probe Go After Pluto?

The historic flyby may be over, but the spacecraft should still go on to study even smaller bodies on its path through the Kuiper belt

Exaggerating the colors on Pluto and Charon helps mission scientists see distinct terrains on each icy world. NASA/JHUAPL/SwRI

Space exploration is often an exercise in delayed gratification. When the New Horizons spacecraft started its voyage to Pluto in 2006, Twitter had just made its public debut. Now, almost a decade later, social media is awash with gorgeous close-ups of the Pluto system, which is turning out to be more textured and complex than anyone imagined.

The closest part of the spacecraft's visit was brief, just a swoop past Pluto's sunlit face that lasted mere hours. But on-board instruments managed to capture a mountain of data that scientists will be sifting through for years, including signs of large impact craters, multicolored terrain and a dusting of Plutonian atmosphere on the poles of the large moon Charon. The first taste of high-resolution data from the flyby is expected to debut this afternoon.

"New Horizons has sent back and will continue to return the most detailed measurements ever taken of Pluto and its system," NASA administrator Charlie Bolden said in the euphoric moments after the team received word that New Horizons had safely completed its close flyby. "It's a historic win for science and for exploration." So with mission scientists hard at work on Earth, what will New Horizons do now that Pluto is in its rear-view mirror?

For the rest of its operational life, the spacecraft will be barreling on through a region of space called the Kuiper belt, a reservoir of cold, icy bodies on the outskirts of the solar system. In late August, mission managers will select a potential follow-up target: a small Kuiper belt object (KB) in the right orbital spot for a possible rendezvous. These objects are some of the oldest, most pristine nubbins of ice and rock in the solar system—leftovers from the process that formed our cosmic neighborhood some 4.6 billion years ago.

"This would be totally unexplored territory. We've never been close to any of these smaller objects in the Kuiper belt," says mission scientist John Spencer of the Southwest Research Institute. "In the Kuiper belt, the original building blocks of the solar system are still out there, many in the locations where they formed. We can see that record in these smaller objects."

Pluto is also a KBO—the largest one known—and that's actually why it's not as good of a record of the solar system's past, says Casey Lisse, a mission scientist at the Johns Hopkins University Applied Physics Laboratory (APL). "Pluto is so big that it has altered itself from when it first formed, it densified and contracted," he says. "How we see that is because it's round—it's big enough to have coalesced by its own self gravity to round out the rough edges." If we want to study the most primordial things in the outer solar system, we need to visit much smaller bodies.

Finding the right targets for an extended mission took a combination of grit and luck. "We would not come close to one by random chance—we definitely needed a target," says Spencer. But if Pluto was just a pixelated orb of light even to the powerful eye of the Hubble Space Telescope, how could anyone hope to find images of more distant objects a fraction of its size?

To the scientists' relief, in October 2014 the search team announced that they had spotted three promising options about a billion miles beyond the Pluto system. Two of the objects are brighter and so are probably bigger; early estimates put them both around 34 miles wide. The third option is smaller, maybe about 15 miles wide, but it would be easier to reach after the Pluto encounter.

"One criteria for selecting the target will be fuel," says Curt Niebur, lead program scientist for NASA's New Frontiers program, which funded the New Horizons mission. A course correction requires a big burn of fuel, so the team has to decide on a target and orient the spacecraft by late October or early November to ensure a safe arrival in 2018.

No matter which KBO makes the cut, New Horizons would then give us an unprecedented look at the landscape in this frigid frontier. "We'll only fly close to one KBO, but we'll observe maybe a dozen from a distance," says Spencer. "We'll be looking for moons, looking at the brightness from different angles, so we'll be exploring other objects, but not in nearly the detail as the main target."

This follow-up mission is not yet a given: the Pluto flyby was the primary point of New Horizons, and the team must apply for more funding to extend their science to a small KBO. On the off-chance that the extension doesn't come through, the New Horizons science team will still be collecting information about the waning breezes of the solar wind in this distant region of space, akin to the magnetic and plasma data being still being collected by the two Voyager probes. Voyager 2 may even serve as a guide for New Horizons as it explores the heliosphere, the bubble of solar material that cocoons our solar system as we hurtle through the galaxy.

Launched in August 1977, Voyager 2 sped past Uranus and Neptune before continuing deeper into the heliosphere. It even crossed near Pluto's orbit in 1989, but aiming for a visit would have meant flying through Neptune—obviously, not an option. Now Voyager 2 is about 9.9 billion miles from Earth, in the outer part of the solar bubble called the heliosheath, and it's still transmitting data. New Horizons will be following a similar path into the mysterious fringes of the solar system.

"It's very fortuitous that New Horizons is in about same heliospheric longitude as Voyager 2," says mission scientist Ralph McNutt at APL. "Even though Voyager 2 is a lot farther out, we kind of have an upstream monitor." As with the Voyager probes, the data returned from New Horizons should help scientists better understand what happens when the solar wind starts to fade and interstellar space takes over—important clues to how the heliosphere shields us from damaging high-energy particles known as galactic cosmic rays. New Horizons probably won't make it to the very edge of the bubble before it runs out of fuel, but it will contribute valuable science for decades to come.

"We should have power until the 2030s, so we can get into the outer part of the heliosphere," says Spencer. "As long as we can continue to get good data—and persuade NASA to pay for it—we will keep getting the data, because we will be in a unique environment that we've never been in before." 

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